In motor vehicles having hydraulically regulated automatic shift transmissions, the various pressure control elements are sized (calibrated) relative to the nominal torque characteristic of the engine to yield acceptable shift quality for various driving and performance conditions. However, the range of variations can be quite broad, especially when the vehicle is operated at differing altitudes. As a result, a vehicle calibrated to deliver high quality shifting at relatively low altitudes may experience excessively harsh shifting at relatively high altitudes. Conversely, a vehicle calibrated to deliver high quality shifting at relatively high altitudes may experience excessive wearing of the clutch elements at relatively low altitudes. The vehicle is particularly sensitive to such variations when the shift involves the control of two clutch elements--that is, a clutch-to-clutch shift.
To overcome the engineering trade-off described above, it has been proposed to equip the transmission with relatively sophisticated electronic pressure control devices, and to actively control the clutch pressures in the course of a shift. While this approach can be used to successfully compensate for driving and performance variations, it often introduces stability concerns, and is not achieved without substantial expense.